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Subject: Re: [PATCH] U300 sched_clock implementation
From: Peter Zijlstra <peterz@infradead.org>
To: Linus Walleij <linus.ml.walleij@gmail.com>
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Date: Mon, 25 May 2009 15:20:27 +0200
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On Mon, 2009-05-25 at 15:01 +0200, Peter Zijlstra wrote:
> On Mon, 2009-05-25 at 14:13 +0200, Linus Walleij wrote:
> > 2009/5/24 Peter Zijlstra <peterz@infradead.org>:
> > 
> > > On Sat, 2009-05-23 at 23:46 +0200, Linus Walleij wrote:
> > >
> > >> This overrides the global sched_clock() symbol in the Linux
> > >> scheduler with a local implementation which takes advantage of
> > >> the timesource in U300 giving a scheduling resolution of 1us. The
> > >> solution is the same as found in the OMAP2 core code.
> > >
> > > We assume sched_clock() to return time in ns (e-9) resolution.
> > 
> > Yep okay and in this case:
> > 
> > >> +       ret = (unsigned long long) u300_get_cycles();
> > >> +       ret = (ret * clocksource_u300_1mhz.mult_orig) >>
> > >> +               clocksource_u300_1mhz.shift;
> > >> +       return ret;
> > 
> > (mult_orig >> shift) == 1000
> 
> Ah, ok -- missed that little detail ;-)
> 
> > So for each cycle in cyclecount register we return 1000 * cycles
> > i.e 1000ns.
> > 
> > If it looks nicer we can of course simply:
> > return (unsigned long long) u300_get_cycles * 1000;
> > 
> > But the question here is whether this resolution is enough for
> > sched_clock() or if it is irrelevant to override sched_clock()
> > if it cannot schedule with better precision than 1000 ns.
> 
> No anything better than jiffies is good, 1us certainly is worth the
> trouble.

One note, sched_clock() is assumed to be _cheap_. Now I assume you knew
that and chose a suitable clocksource.

But that is the reason this isn't generic, non of the 'stable'
clocksources on x86 are fast enough to use as sched_clock.

Of course, x86 isn't the only arch and if enough architectures do show
this pattern, we could indeed think about doing this in generic code.